1. Field of the Invention
The present invention relates to a meter driving device and particularly to such a device for driving a cross-coil meter that electrically energizes two coils extending perpendicular to each other to generate a magnetic field which provides a desired azimuth.
2. Description of the Invention
There is known such a cross-coil meter that electrically energizes two exciting coils extending perpendicular to each other to create a magnetic field which in turn provides a torque to a rotatably supported magnet to indicate a desired azimuth. These cross-coil meters are currently used in speed meters, tachometers and other meters in motorcars and so on.
FIGS. 4 and 5 show the schematic arrangement of a cross-coil meter according to the prior art.
The cross-coil meter comprises two exciting coils L.sub.S and L.sub.C which are arranged to extend perpendicular to each other. When these exciting coils are energized by an electric current depending on an input, such as from a motorcar speed meter, they generate a magnetic field corresponding to a desired azimuth. The rotatably supported permanent magnet is rotated by a torque from the magnetic field generated by the two exciting coils L.sub.S and L.sub.C.
An indicating arm is fixedly mounted on the permanent magnet. The indicating arm is angularly shifted from a reference position by an angle 8 such that it indicates a given position on a meter indicating plate on which is printed a physical quantity to be measured. For example, as shown in FIG. 5, vehicle speeds may be printed on the meter indicating plate.
A device for driving such a cross-coil meter generally receives pulse signals having frequencies which are variable depending on the amount of input (e.g. vehicle speed). The pulse signals are measured by counting the number of pulses in reference clock signals which are generated by a reference clock generating means. More particularly, if it is assumed that input pulse signal cycles are T.sub.1, T.sub.2, T.sub.3 and so on, each of these cycles is measured by counting the number of pulses of the reference clock signals to compute frequencies F=1/T.sub.1, F=1/T.sub.2, F=1/T.sub.3 and so on.
Electric currents corresponding to these computed frequencies are subjected to pulsewidth modulation before they are provided to the exciting coils L.sub.S and L.sub.C which are arranged to extend perpendicularly to each other.
The meter driving device of the prior art raises a problem from the fact that the cycles T of input pulse signals are measured with their inverse numbers being then computed to determine input frequencies F=1/T, as described. For example, if the device is used to drive a speed meter, the input cycle is very prolonged during low-speed running (several kilometers per hour) with its frequency becoming several Hertz. Since the frequency is updated after an increased period of time, the indicating arm will be moved or shifted intermittently and will not provide a smooth indication.
If any modification is made to provide a smooth indication in the basic structure of the prior art, another problem is raised in that the whole arrangement becomes complicated and the scale of the circuit is increased.